時間・時点N2
〜（か）と思うと、〜（か）と思ったら
just when... then... / one moment... the next...
Structure
V-た形／辞書形 + （か）と思うと／と思ったら
Explanation
① two actions happen almost simultaneously ("just when..., then..."); ② states alternate ("one moment..., the next..."). Attached to the ta-form/dictionary form of verbs. Similar to かと思えば.
Examples
笑ったかと思うと、泣き出した。
One moment she was laughing, the next she burst into tears.
あの子は遊んでいるかと思ったら、もう宿題を始めている。
Just when I thought the child was playing, he's already started his homework.
Note
Basically synonymous with かと思えば. In sense ② it often pairs with また/もう for rapid alternation.